As the era of multimedia has progressed, so has the rapid spread of display devices. These find use in small-size applications such as the viewfinders of projectors and video cameras as well as cellular phones, in mid-size applications such as the display panels of vehicular televisions and navigation systems as well as mobile terminals such as personal digital assistants (PDAs) and pocket personal computers, and in large-size applications such as notebook personal computers and monitors. Among these display devices, liquid crystal display devices presently are being applied to the largest group of products. In particular, active-matrix liquid crystal devices driven by thin-film transistors (abbreviated to “TFT” below) are the dominant liquid crystal display devices because they exhibit a resolution and image quality that are superior to those of simple matrix-type liquid crystal display devices. TFTs are classified as amorphous silicon TFTs and polysilicon TFTs depending upon a difference in the semiconductor material used.
Amorphous silicon TFT does not require a high-temperature fabrication process. This makes it possible to fabricate a panel using a substrate such as glass.
Because polysilicon TFTs conventionally require a high-temperature process, they necessitate expensive quartz substrates and are limited to small-size panels of high added value. Owing to advances in techniques such as laser annealing in recent years, technology has been developed that makes it possible to form a precursor film by low-pressure (LP) CVD, plasma (P) CVD or sputtering, etc., subject the film to polycrystallization by laser annealing and form a polysilicon TFT at low temperature that allows use of a glass substrate or the like. Mid-size display panels and display panels for notebook personal computers also can now be fabricated using polysilicon TFTs.
In comparison with amorphous silicon TFT, a polysilicon TFT has a mobility that is higher by an order of magnitude and exhibits a higher current driving capability.
When a liquid crystal display device is constructed using polysilicon TFTs, the fact that such a TFT has a high current driving capability enables the integration of peripheral circuitry on the same substrate as the pixels. As a consequence, it is possible to realize a reduction in the number of LSI elements, a reduction in size and a reduction in packaging cost.
A liquid crystal display device in which peripheral circuitry is integrated with the same substrate as the pixels is referred to as a “combined driver circuit and liquid crystal display device”.
The most popular type of combined driver circuit and liquid crystal display device has, as the peripheral circuitry, a data driver that drives the data line connected to the source terminals of the pixel TFTs, and a gate driver that drives the gate lines connected to the gate terminals of the pixel TFTs. Such liquid crystal display devices find wide use in liquid crystal projectors, which require small, high-definition LCDs, and in portable notebook personal computers that require a picture frame of reduced size.
With a driver device in a conventional liquid crystal display in which the driver circuits are not integrated with the substrate, a group of gate driver LSI chips, a controller and a DC-DC converter are provided on a TCP (Tape Carrier Package) and a flexible circuit board or connection circuit board. With this structure, packaging becomes more complicated as definition and tonality increase, and an increase in the size of the picture frame cannot be avoided. At the same time, the problem of EMI (Electromagnetic Interference) becomes more pronounced owing to higher frequency. For this reason, great endeavors have been made to deal with the noise problem. These include reinforcing the ground wiring of the printed circuit board used, altering the arrangement of component materials on the printed circuit board, changing the routing of wiring, adding on EMI filters and improving interfaces.
By contrast, the integrated type of driver circuits in which the peripheral circuits are integrated on the same substrate lends itself to easy packaging and the size of the picture frame does not change much even if higher definition and tonality are provided. Such a device is extremely effective for use in mobile applications.
FIG. 37 is a diagram illustrating an overview of a display system that employs a liquid crystal display device integral with driver circuits according to the prior art. In this conventional combined driver circuit and liquid crystal display device, as shown in FIG. 37, an active-matrix display area 110, in which pixels of M rows and N columns are arranged the form of a matrix, a row-direction scanning circuit [scanning-line (gate-line) driver circuit] 109, a column-direction scanning circuit (data-line driver circuit) 3504, an analog switch 3505 and a level shifter 3503 are formed integrally by polysilicon TFTs on a display device substrate 101.
A controller 113, a memory 111, a digital/analog converter (DAC) 3502, a scanning-line/data register 3501 and an interface circuit 114, etc., are formed external to the display device substrate 101 using monocrystalline silicon circuits (LSI circuits).
The analog switch 3505 has outputs the number of which is the same as the number N of column-direction data lines of the active-matrix display area 110.
The conventional combined driver circuit and liquid crystal display devices also include devices of the type having more complicated built-in circuits, such as DACs. FIG. 38 is a diagram illustrating an overview of a display system that employs a liquid crystal display device integral with driver circuits and having a built-in DAC according to the prior art. In the conventional liquid crystal display device having the built-in DAC, the following circuits are formed on the display device substrate 101 in addition to the active-matrix display area 110, in which pixels of M rows and N columns are wired in the form of a matrix, the row-direction scanning circuit 109 and a column-direction scanning circuit 3506 similar to those of the device in FIG. 37 not having the built-in DAC: a data register 3507, a latch circuit 105, a DAC circuit 106, a selector circuit 107, a level shifter/timing buffer 108 and a level shifter.
According to this arrangement, the controller IC having an internal memory does not include the DAC; the memory 111, an output buffer 112 and the controller 113 are all implemented by digital circuits. As a result, fabrication is possible without making joint use of a process for analog circuits. This means that the IC can be fabricated at a cost lower than that the above-mentioned driver IC having the internal memory.
The liquid crystal display device set forth above is thin and light and consumes less power than a CRT (cathode-ray tube). This feature is exploited to mount the liquid crystal display device on mobile information processing equipment.
Owing to the rapid spread of mobile terminals such as cellular phones, PDAs and mobile personal computers in recent years, there is increasing demand for displays used in mobile applications. A display for use in such mobile terminals must satisfy the following requirements:
(a) The area of the device, with the exception of the display, must be reduced in order to enhance portability.
(b) Mobile terminals generally are powered by batteries. Low power consumption is desired, therefore, in order to prolong continuous operating time provided by a single charge.
(c) Since a low price is necessary in order for mobile terminals to become more widespread, it is desired that mobile displays also be reduced in cost.
It is expected that these requirements can be implemented by a combined driver circuit and liquid crystal display device and by an organic EL (electroluminescence) device, etc.
The specification of Japanese Patent Kokai Publication JP-A-11-202290 discloses a device so adapted as to lower the power consumption, reduce the size and improve the definition of a liquid crystal display having built-in peripheral circuits. The device is such that a peripheral circuit on the signal side and a peripheral circuit on the scanning side for driving liquid crystal, as well as a connecting portion having a relay bus for transferring display data to signal wiring, are formed on a TFT substrate, and an image memory chip, which is formed to include a read-out control circuit and an image memory for storing at least one line of image data read in from a CPU via the connecting portion, is mounted on a liquid crystal display device. Display data from the image memory chip is transferred in parallel one line at a time in response to a low-speed clock.